using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string input = "Batch K1 Forms - 2019-05-02_02-57-41-PM.zip - 2019-05-04_10-09-28-AM.zip";

        string output = Test(input);

        Console.WriteLine(output);

    }

    private static readonly Regex Pattern = new Regex(@"\.(pdf|zip|xml|png)", RegexOptions.Compiled);

​

    public static string Test(string input)

    {

        return Pattern.Replace(input, match =>

        {

            string newMatch = match.Groups[1].Value;

            return ">> " + newMatch + " <<";

        });

    }

}